docs: def _metric_when_multiplied_with_sig_vec(self, sig):
          """return D^-1 B^T diag(sig) B D as a measure for
          C^-1/2 diag(sig) C^1/2
  
          :param sig: a vector "used" as diagonal matrix
          :return:
  
          """
          return dot((self.B * self.D**-1.).T * sig, self.B * self.D)